Optoelectronic sensor and method for detecting objects

ABSTRACT

An optoelectronic sensor is provided that has at least one light transmitter for transmitting a plurality of mutually separated light beams starting from a respective one transmission point; a transmission optics for the transmitted light beams; at least one light receiver for generating a respective received signal from the remitted light beams reflected from the objects and incident at a respective reception point; a reception optics for the remitted light beams; and an evaluation unit for acquiring information on the objects from the received signals. The reception optics and/or the transmission optics is/are a two-lens objective for an annular image field with an image field angle that has a first lens and a second lens, with the first lens being configured such that bundles of rays of every single transmission point and/or reception point with an image field angle α only impinge on half of the second lens.

The invention relates to an optoelectronic sensor and to a method fordetecting objects in a monitored zone in accordance with the preamblesof the respective independent claim.

Many optoelectronic sensors work in accordance with the sensingprinciple in which a light beam is transmitted into the monitored zoneand the light beam reflected by objects is received again in order thento electronically evaluate the received signal. The time of flight ishere often measured using a known phase method or pulse method todetermine the distance of a sensed object.

To expand the measured zone of a single-beam light scanner, the scanningbeam can be moved, on the one hand, as is the case in a laser scanner. Alight beam generated by a laser there periodically sweeps over themonitored zone with the help of a deflection unit. In addition to themeasured distance information, a conclusion is drawn on the angularlocation of the object from the angular position of the deflection unitand the location of an object in the monitored zone is thus detected intwo-dimensional polar coordinates.

Another possibility for extending the measured zone and for acquiringadditional distance data comprises simultaneously detecting a pluralityof measured points using a plurality of scanning beams. This can also becombined with a laser scanner that then does not only detect a monitoredplane, but also a three-dimensional spatial zone via a plurality ofmonitored planes. The scanning movement is achieved by a rotating mirrorin most laser scanners. Particularly on the use of a plurality ofscanning beams, however, it is also known in the prior art to insteadhave the total measurement head with the light transmitters and lightreceivers rotate, as is described, for example, in DE 197 57 849 B4.

In principle, a plurality of scanning beams can be achieved by amultiplication of the elements of a single-beam device. However, this isunnecessarily expensive and complex. There are therefore approaches inthe prior art to use elements multiple times. In DE 10 2015 121 839 A1,for instance, the scanning beams of a plurality of light transmittersare formed by a common transmission optics and are deflected in thedesired direction.

It is now admittedly desirable to use the transmission optics or alsothe reception optics simultaneously for a plurality of beams. However,contradictory demands are made on the optics here. It should, on the onehand, be as simple as possible for cost reasons, but should image allthe beams in focus simultaneously in an image field that is as large aspossible.

A simple optics can be implemented with the aid of a single lens.However, only a small image field angular range of at most ±5° ispossible with it under the given conditions of a sufficiently largeaperture, for example with an f-number k≤3 and with small dot images.Only densely disposed scanning beams can consequently be implementedwith a single lens. With a sensible mutual interval of some degrees andwith said image field angles of <5°, a single lens does not permit morethan two or three scanning beams and thus does not permit any largermeasured zones of, for example, 30°.

On the other hand, there is also the possibility of dispensing with asimple optics and of using a multi-lens objective. Small dot images canalso be generated with them over a larger image field angle of, forexample, ±20°. However, as a rule, at least three lenses that also haveto be adjustable per se are required for this purpose.

Even if the higher manufacturing and adjustment costs are accepted,optical compromises still also have to be made. While f-numbers of k=1can substantially only be achieved at 0° along the optical axis for animage field with single lenses, an f-number of k≤3 or even k≤2 can onlybe achieved with difficulty with objectives for a large image field of,for example, ±15°. However, the range of the device is reduced with asmaller receiving aperture. A further disadvantage of a multi-lensobjective is that the main beam is incident on the image plane with afairly large angle. If such an objective is used as the transmissionoptics, it becomes necessary to tilt the light sources to be incident onthe objective at all. The alternative of a telecentric objective inwhich the main beam angle remains very small cannot be consideredbecause this then requires a lot more than three lenses.

It is therefore the object of the invention to simplify and improve amulti-beam system of the category.

This object is satisfied by an optoelectronic sensor and by a method fordetecting objects in a monitored zone in accordance with the respectiveindependent claim. The sensor in accordance with the invention is amulti-sensor that transmits a plurality of light beams from onerespective transmission point by at least one light transmitter. Eachtransmission point is effectively a light transmitter from which therespective transmitted light beam originates, with it, however,initially not being absolutely necessary that physical lighttransmitters are located at the transmission points. Instead, in someembodiments, one physical light transmitter can also generate aplurality of transmitted light beams at a plurality of transmissionpoints or at all the transmission points, as will be explained below.The transmitted light beams are furthermore not to be understood as raysin the sense of geometrical optics within a larger bundle of rays, butrather as mutually separate bundles of rays and thus isolated scanningbeams that generate correspondingly isolated, mutually spaced apartlight spots in the monitored zone on impinging onto an object.

At least one light receiver is able to generate a respective receivedsignal from the light beams that are remitted from different directions,that are reflected at the objects, and that are incident at a respectivereception point. In a similar manner to the statements on thetransmission points, the reception points are effectively lightreceivers without a light receiver physically having to be located ateach reception point. Received signals generated in this manner areevaluated to acquire information on the object.

The transmitted light beams pass through a transmission optics that, forexample, provides that the transmitted light beams have a desired beamshape, a clear separation from one another, or specific irradiationdirections. Only one transmission optics is provided here for all thelight beams of all the transmission points. It is, however, conceivableto combine a plurality of modules that each have a plurality oftransmission points and only one transmission optics for a sensor havinga very large number of beams. The same applies accordingly to theremitted light beams and to their common reception optics.

The invention starts from the basic idea of using a two-lens objectivefor a circular image field with an image field angle α as thetransmission optics and/or reception optics. A two-lens objectivecomprises exactly two lenses, that is a first lens and a second lens andno further lens, with the lenses in particular being converging lenses.The first lens is shaped such that incident bundles of rays for everysingle image point or transmission point and/or reception point with animage field angle α are only incident on a respective half of the secondlens. Such bundles of rays, and indeed preferably all the bundles ofrays with an image field angle α, are therefore only located at thelevel of the second lens at one respective side of the optical axis.Bundles of rays from field points disposed opposite with respect to theoptical axis do not overlap one another in the plane of the second lens.They just no longer illuminate the center of the second lens in thismanner. Unlike conventional objectives, no large contiguous range ofimage field angles are therefore used here such as—α . . . 0° . . . α,but only a single discrete image field angle α. The image field angle αcan include a certain tolerance environment within which theseproperties are still just sufficiently satisfied.

The invention has the advantage that useful properties of both a singlelens and of an objective are combined using the specific two-lensobjective design. A two-lens objective is naturally simpler to produceand to adjust than a three-lens or multi-lens objective. The degrees ofdesign freedom are in turn more restricted, but are utilized inaccordance with the invention such that a plurality of scanning beamsare imaged in focus over a larger image field despite the smallereffort. Field angles of, for example, ±20° are possible withoutincreasing the dot images that satisfy a demand such as <0.5 mrad. Alarge number of mutually spaced apart beams thus become possible withthe same optics and in this respect the performance is not less thanwith a three-lens or multi-lens objective. This is possible in thatsharply delineated beams are not required everywhere, but only on anannulus corresponding to the image field angle α. The two-lensobjective, however, unlike a multi-lens objective, still enables a largeaperture of k≤2 or even of k=1, that is a small f-number and thus alarge aperture and a large range. It is even possible to keep the mainbeam angle very small in the image plane and thus to design an objectivethat is almost telecentric at the image side. On a use in thetransmission path, the beam sources then do not have to be tilted, butcan be soldered on a circuit board in a flat manner.

The inequality d≥(D1*f1)/(D1+2*f1*tan α) is preferably satisfied for thetwo-lens objective, with a focal length f1 and a diameter D1 of thefirst lens and a distance d between the first lens and the second lens.The distance d is here preferably measured between the main plane of thefirst lens and the first effective lens surface of the second lens, thatis that lens surface that faces the first lens. This is a mathematicalformulation for the already presented condition that lights beams havingan image field angle α are only incident on half of the second lens.

Equality, i.e. d=(D1*f1)/(D1+2*f1*tan α), preferably applies at leastapproximately. At least approximately means that a certain tolerance of,for example, 5% or 10% is still possible and the properties of theoptics do not change abruptly. It is rather a question of stillobserving a distance of the second lens from the image plane of theoptics that is as large as possible under the condition of theinequality so that the second lens can also develop a still noticeableeffect. The equality is here an optimum in the sense that this distanceis maximized and this optimum can also only be approximately reachedwith said tolerance.

The focal length f2 of the second lens preferably corresponds to thedistance between the first lens and the second lens. The two-lensobjective then becomes telecentric at the image side. This inter aliaallows, as already addressed, the beam sources to be aligned in parallelwith one another on a use as a transmission optics. The distance betweenthe first lens and the second lens is preferably not the alreadyintroduced distance d at this point, but rather the distance d′ betweenthe main planes of both lenses different by approximately half thecenter thickness of the second lens. This does not, however, necessarilyhave to be observed in this manner. In a similar manner to thatdiscussed in the previous paragraph, a focal length of f2 approximatelysimilar to the distance between the first and second lenses also alreadybrings about advantages; the two-lens objective is then at least almosttelecentric at the image side and at least one tolerance can easily beaccepted within the framework of half the center thickness.

The first lens preferably has a small f-number k1, in particular k1=1.It is an advantage of the two-lens objective with respect to multi-lensobjectives that this is possible at all. This selection of k1 alsoresults in a small value k for the f-number of the total objective. Aparticularly sensitive sensor with a long range can be implemented withsuch a small f-number.

The transmission points are preferably arranged on a first circular lineand/or the reception points are arranged on a second circular line. Theannuli correspond to the image field angle α for which the two-lensobjective is designed. With this mutually matching arrangement oftransmission points or reception points and the two-lens objective, itsoptimized properties are actually used while design compromises possiblyaccepted in turn no longer have any effect for other image field angles.A circular arrangement of the transmission points and reception pointsin particular seems counterproductive at first glance for a laserscanner since a one-dimensional arrangement on a simple line with whicha plane group is then scanned by a rotational movement would besufficient there. However, the line, unlike an annulus, could not beimaged over the required large image range by a two-lens object with thehigh quality in accordance with the invention. In addition, it is alsopossible with transmission points and reception points arranged to forma circular line to scan planes arranged equidistantly in a laser scannersince the offset in the direction of rotation of the laser scanner onlyeffects a time offset of the measurement that can also be compensated ifrequired.

The first circular line is preferably centered about the optical centeraxis of the transmission optics and/or the second circular line iscentered about the optical center axis of the reception optics. In otherwords, the optical center axes of the respective optics extend throughthe circle center of the circular line. The light beams then eachundergo the same beam shaping effects and deflection effects withrotationally symmetrical properties of the optics.

The transmission points are preferably evenly distributed over the firstcircular line and/or the reception points are evenly distributed overthe second circular line. Such an even arrangement in which thetransmission points or reception points form a regular n-gon is simplerto handle, in particular to obtain equal angular intervals between thescanning beams. An irregular distribution over the circular line,however, also remains possible and this in no way precludes neverthelesssetting equal angular intervals between the scanning beams.

Three or more transmission points or reception points are preferablyprovided. Five, six, eight, or sixteen can be named as particularlyadvantageous. With a number of four, the transmission points and/or thereception points are preferably not arranged to form a square or arectangle. It must be repeated that the number can also relate to atransmission/reception module of which a plurality can be installed inan optical sensor. The total number of scanning beams is then added upfrom the numbers of installed modules and further numbers can thus beconstructed.

The sensor preferably has a plurality of light transmitters or lightreceivers, in particular one light transmitter per transmission pointand/or a plurality of light receivers or light reception elements, inparticular one light receiver per reception point. The transmitted lightbeams are therefore generated in part, if not in full, directly by theirown light transmitters at the transmission points and the same appliesaccordingly to the remitted light beams, to the reception points and tothe light receivers.

A beam splitter element is preferably associated with the lighttransmitter to split its light into a plurality of transmitted lightbeams. A single physical light transmitter is thus responsible for aplurality of transmission points or even for all the transmissionpoints. It is also conceivable to split the light of a plurality ofphysical light transmitters, for example to obtain six transmissionpoints from two light transmitters with a light beam split three times.

The light receiver is preferably spatially resolved and has a pluralityof active zones at the reception points. In this embodiment, the samelight receiver is responsible for a plurality of reception points oreven for all the reception points. For this purpose, the light receiverin particular has a pixel matrix and only the pixel or pixels at thereception points is/are used to acquire a received signal. The remainingpixels possibly also generate signals that can, however, be ignored ornot read. It is also conceivable to fully deactivate such pixels, forinstant to directly only bias the pixels at the reception points abovethe breakdown voltage in a SPAD (single photon avalanche diode) matrix.

The light transmitters and light receivers preferably form a coaxialarrangement and the transmission optics and the reception optics arecombined in a common optics. This produces a particularly compactdesign. There is then in total only one two-lens objective as the commonoptics that acts in a dual role as a transmission and reception optics.

The sensor is preferably configured as a laser scanner and has a movingdeflection unit with whose aid the transmitted light beams are conductedperiodically through the monitored zone. As explained in theintroduction, the laser scanner scans the monitored zone in a pluralityof zones with the movement of the moving deflection unit. A greaterplane distance or a greater spatial angular range covered overall by thescanning planes is made possible in elevation by the large image fieldof the two-lens objective. The deflection unit is preferably configuredin the form of a rotatable scanning unit that practically forms amovable measurement head in which at least the light transmitter opticaltogether with the common transmission optics and possibly also the lightreceiver and least parts of the evaluation unit are accommodated.

The evaluation unit is preferably configured to determine a distance ofthe object from a time of flight between the transmission of the lightbeams and the reception of the remitted light beams. The sensor therebybecomes distance measuring. Alternatively, only the presence of anobject is determined and is output as a switching signal, for example.

The method in accordance with the invention can be further developed ina similar manner and shows similar advantages in so doing. Suchadvantageous features are described in an exemplary, but not exclusivemanner in the subordinate claims dependent on the independent claims.

The invention will be explained in more detail in the following alsowith respect to further features and advantages by way of example withreference to embodiments and to the enclosed drawing. The Figures of thedrawing show in:

FIG. 1 a schematic sectional representation of a laser scanner;

FIG. 2a a schematic view of a circular arrangement of image fieldpoints;

FIG. 2a a schematic view of a linear arrangement of image field points;

FIG. 2c a schematic view of an annular arrangement of image fieldpoints;

FIG. 3 a plan view of circularly arranged transmission points andreception points;

FIG. 4 a schematic view of a two-lens objective for an annular imagefield with exemplary beam progressions; and

FIG. 5 a schematic plan view of the second lens of the objective inaccordance with FIG. 4 to illustrate the optical effect of the firstlens.

FIG. 1 shows a schematic sectional representation through anoptoelectronic sensor 10 in an embodiment as a laser scanner. The sensor10 in a rough distribution comprises a movable scanning unit 12 and abase unit 14. The scanning unit 12 is the optical measurement head,whereas further elements such as a supply, evaluation electronics,terminals and the like are accommodated in the base unit 14. Inoperation, the scanning unit 12 is set into a rotational movement aboutan axis of rotation 18 with the aid of a drive 16 of the base unit 14 tothus periodically scan a monitored zone 20.

In the scanning unit 12, a light transmitter 22 having a plurality oflight sources 22 a, for example LEDs or lasers in the form of edgeemitters or VCSELs, generates with the aid of a common transmissioncommon optics 24 a plurality of transmitted beams 26 having a mutualangular offset that are transmitted into the monitored zone 20. If thetransmitted light beams 26 impact an object in the monitored zone 20,corresponding remitted light beams 28 return to the sensor 10. Theremitted light beams 28 are conducted from a common reception optics 30to a light receiver 32 having a plurality of light reception elements 32a that each generate an electric received signal. The light receptionelements 32 a can be separate elements or pixels of an integrated matrixarrangement, for example photodiodes, APDs (avalanche diodes), or SPADs(single photon avalanche diodes).

The purely exemplary four light sources 22 a and light receptionelements 32 a are shown above one another in the sectional view. Inactual fact, at least one of the groups is arranged in preferredembodiments of the invention in a circular figure or on a circular line,as will be explained further below. However, this does not have torelate to physical light sources 22 a and light reception elements 32 a,but only to the effective transmission points, that do, however, agreetherewith here, as starting points of the transmitted light beams 26 andto reception points as end points of the remitted light beams 28.Differing from FIG. 1, it is conceivable to generate a plurality oftransmission points with a physical light source or to accommodate aplurality of reception points on the same physical reception module.

The light transmitter 22 and the light receiver 32 are arranged togetherin the embodiment shown in FIG. 1 on a circuit board 34 that is disposedon the axis of rotation 18 and that is connected to the shaft 36 of thedrive 16. This is only to be understood by way of example; practicallyany desired numbers and arrangements of circuit boards are conceivable.The basic optical design with a light transmitter 22 and a lightreceiver 32 biaxially disposed next to one another is also notcompulsory and can be replaced with any construction design known per seof single-beam optoelectronic sensors or laser scanners. An example forthis is a coaxial arrangement with or without a beam splitter.

A contactless supply interface and data interface 38 connects the movingscanning unit 12 to the stationary base unit 14. A control andevaluation unit 40 is located there that can at least partly also beaccommodated on the circuit board 34 or at another location in thescanning unit 12. The control and evaluation unit 40 controls the lighttransmitter 22 and receives the received signals of the light receiver32 for further evaluation. It additionally controls the drive 16 andreceives the signal of an angular measurement unit which is not shown,which is generally known from laser scanners, and which determines therespective angular position of the scanning unit 12.

The distance from a scanned object is measured for the evaluation,preferably using a time of flight process. Together with the informationon the angular position from the angular measurement unit,two-dimensional polar coordinates of all the object points in a scanningplane are available after every scanning period with angle and distance.The respective scanning plane is likewise known via the identity of therespective transmitted light beam 28 and its detection in one of thelight reception elements 32 a so that a three-dimensional spatial zoneis scanned overall.

The object positions or object contours are thus known and can be outputvia a sensor interface 42. The sensor interface 42 or a furtherterminal, not shown, conversely serves as a parameterization interface.The sensor 10 can also be configured as a safety sensor for use insafety engineering for monitoring a hazard source such as a dangerousmachine. In this process, a protected field is monitored which may notbe entered by operators during the operation of the machine. If thesensor 10 recognizes an unauthorized intrusion into the protected field,for instance a leg of an operator, it triggers an emergency stop of themachine. Sensors 10 used in safety technology have to work particularlyreliably and must therefore satisfy high safety demands, for example thestandard EN13849 for safety of machinery and the machinery standardEN1496 for electrosensitive protective equipment (ESPE). The sensorinterface 42 can in particular be configured as a safe output device(OSSD, output signal switching device) to output a safety-directedswitch-off signal on an intrusion of a protected field by an object.

The sensor 10 shown is a laser scanner having a rotating measurementhead, namely the scanning unit 12. In this respect, not only atransmission/reception module can rotate along as shown here; furthersuch modules with a vertical offset or an angular offset with respect tothe axis of rotation 18 are conceivable. Alternatively, a periodicdeflection by means of a rotating mirror or by means of a facet mirrorwheel is also conceivable. With a plurality of transmitted light beams26, it must, however, be noted that how the plurality of transmittedlight beams 26 are incident into the monitored zone 20 depends on therespective rotational position since their arrangement rotates by therotating mirror as known geometrical considerations reveal. A furtheralternative embodiment pivots the scanning unit 12 to and fro, eitherinstead of the rotational movement or additionally about a second axisperpendicular to the rotational movement to also generate a scanningmovement in elevation.

The embodiment as a laser scanner is also advantageous. A multiplesensor without a periodic movement is also possible that thenpractically only comprises the stationary scanning unit 12 havingcorresponding electronics, but without a base unit 14, in particular asa variant of a flash LIDAR.

During the rotation of the sensor 10, a respective area is scanned byeach of the transmitted light beams 26. A plane of the monitored zone 20is here only scanned at a deflection angle of 0°, that is with ahorizontal transmitted light beam not present in FIG. 1. The remainingtransmitted light beams scan the envelope surface of a cone that isdesigned as differently acute depending on the deflection angle. With aplurality of transmitted light beams 26 that are deflected upward anddownward at different angles, a kind of nesting of a plurality ofhourglasses arises overall as a scanned structure. These envelopesurfaces of a cone are here also sometimes called scanning planes insimplified terms.

In accordance with the invention, the transmission optics 24 and/or thereception optics 30 are configured for an annular image field having animage field angle α. The motivation for this will be explained withreference to FIGS. 2a -c.

In the ideal case, the optics 24, 30 should, as in FIG. 2a , image allthe image field positions 46 in focus within the image circle 44. Inaccordance with the introductory discussion, a single lens, however,only does this for a very small image circle 44, whereas a correspondingobjective would be too complex and would additionally bring about otheroptical limitations.

An areal imaging is not necessarily required for a laser scanner sincescanning planes having a mutual offset in elevation are already producedby a linear arrangement of light sources 22 a and light receptionelements 32 a. One optics would be sufficient for this purpose thatprovides a sharp imaging on a linear arrangement of image fieldpositions 46 as in FIG. 2b . However, this is also only possible forlarger image circles 44 using a complex objective.

Instead, the sharp imaging is only required for a single image fieldangle α such as is shown in FIG. 2c where the ring of the image fieldpositions 46 corresponds to the image field angle α. The optics designis preferably oriented on the fixed image field angle α, which does notpreclude the imaging also still being sharp in a certain environment;however, this is no longer a design demand for differing, and inparticular smaller, image field angles. The image field angle α having acertain tolerance band of a sufficiently sharp imaging is as large aspossible in FIG. 2c , for example α=±15°, to obtain distances that arelarge as possible between the beams 26, 28 of the sensor 10. A certainimprovement of the angle covered, for example to ±8°, can already beachieved by this restriction to an annular image field. This will be,however, be improved considerably more with the configuration of theoptics 24, 30 in accordance with the invention explained below withreference to FIGS. 4 and 5.

FIG. 3 shows in a plan view a preferred arrangement of the light sources22 a or of the light reception elements 32 a on a circular line 48 a-b.As shown, the optical center axis of the optics 24, 30 preferably passesthrough the center of the circular line 48 a-b. FIG. 3 can relate to thetransmission path and/or to the reception path depending on theembodiment so that the reference numerals are shown in dual form.

Due to the arrangement on the circular line 48 a-b, only the annularimage field corresponding to the image field angle α is effectivelyused. This arrangement is therefore particularly advantageous; exactlythe optimized zones of the optics 24, 30 are utilized. Aberrations ofthe optics 24, 30 for image field angles differing from α arepractically irrelevant.

The difference between a light source 22 a and a transmission point 22 bwas already briefly looked at in connection with FIG. 1. A transmissionpoint 22 b is the starting point of a transmitted light beam 26. It cansimultaneously be the location of a physical light source 22 a. On theone hand, however, a light source 22 a as a semiconductor component alsohas a certain basic shape, here a square basic shape, that is largerthan the emission surface itself. It is moreover possible to generatetransmitted light beams from a plurality of transmission points 22 busing one and the same physical light source 22 a. The transmissionpoints 22 b are naturally strictly speaking not mathematical points, butrather have a finite extent so that only some points, and in particularthe center points, can be arranged on the circular line 48 a-b. Thestatements on the transmission points 22 b apply accordingly to thereception points 32 b. The arrangement of the transmission points 22 bor of the reception points 32 b is ultimately relevant for the opticalproperties of the sensor 10, not that of the light transmitters 22,light sources 22 a, light receivers 26, or light reception elements 32a.

In the embodiment in accordance with FIG. 1, each transmission point 22b is implemented by its own light source 22 a and each reception point32 b is implemented by its own light reception element 32 a. It isalternatively possible to deviate from this in the most varied manner.The same light source 22 a can generate transmitted light beams 26 froma plurality of transmission points 22 b or even from all thetransmission points 22 b by a beam splitter element or the like. Thelight source 22 a can be moved mechanically to generate transmittedlight beams 26 consecutively from a plurality of transmission points 22b or even from all the transmission points 22 b. The transmitted lightbeam 26 can also move over the circular line 48 a or over a part thereofwithout a mechanical movement of the light source 22 a, for instance bymeans of a MEMS mirror, an optical phased array, or an acousto-opticalmodulator.

A plurality of reception points 32 b can in turn equally be achieved byseparate light reception elements 32 a such as pixels or pixel zones ofan integrated multiple arrangement of light reception elements 32 a. Amechanical movement of a light reception element 32 a along the circularline 48 a or along a part thereof or a corresponding deflection of theremitted light beams 28 by means of a moved MEMS mirror or the like isalso conceivable on the reception side. In a further embodiment, thereceived light of a plurality of reception points 32 b or of all thereception points 32 b is conducted to a common light reception element.To nevertheless be able to determine the identity of the respectiveremitted light beam 28, a multiplexing is possible with a sequentialactivation of transmitted light beams 26 or via a time encoding of themultiple pulse sequence of the transmitted beams.

FIG. 3 shows an example with three transmission points 22 b or receptionpoints 32 b that are evenly distributed over the circular line 48 a-b.Differing from this, various numbers 3, 4, 5, 6, 7, 8, . . . 16 and moreare also conceivable in an irregular arrangement.

FIG. 4 shows a schematic representation of a two-lens objective having afirst lens 50 and a second lens 52, with both lenses 50, 52 preferablybeing converging lenses. This objective can be used as a transmissionoptics 24 and/or as a reception optics 32. It was explained above withrespect to FIG. 2 that image field angles of up to ±18° are possiblewith a single lens optimized for an annular image field. The two-lensobjective substantially improves this to ±20° and more.

Two exemplary bundles of rays 54, 56 that are disposed opposite oneanother with respect to the optical axis and that correspond to theimage field angle α are drawn in FIG. 4. The two-lens objective is alsooptimized for this image field angle α and the annular image fieldthereby determined.

The first lens 50 reduces the beam diameter of the bundles of rays 54,56 to a cross-section that is at a maximum still half as large as on theentry into the first lens 50. This reduced cross-section then onlyimpinges a half of the second lens 52. The second lens 52 is therebyalways only illuminated by light from one field point at a givenposition, but not from the field point disposed opposite with respect tothe optical axis.

FIG. 5 illustrates these optical properties of the two-lens objectiveagain in a plan view of the second lens 52. Bundles of rays 54, 56 and54′. 56′ of oppositely disposed field points do not overlap and do notreach the respective other half of the second lens 52 with respect tothe optical axis. Laterally adjacent field points may also result in aspecific overlap. The center of the second lens 52 remains withoutillumination.

These qualitatively explained properties can also be indicated moreexactly with reference to the parameters of the two-lens objective. Thatdistance d between the main plane of the first lens 50 and the firstoptically active surface of the second lens 52 is sought in which allthe beams of a bundle of rays 54, 56 to a field point have completelyarrived at one side with respect to the optical axis.

The main beam of the light bundle 50, 52 through the center of the firstlens 50 has a side offset of tan α*d with a variably conceived d and therelevant marginal beam still has an additional lateral offset of(D1/2)f1*d, where D1 is the diameter used and f1 is the focal length ofthe first lens 50. Overall, the lateral offset should move the marginalbeam beyond the optical axis. A lateral offset of d1/2 is necessary forthis. The following inequality therefore has to be satisfied:

[(D1/2)/f1+tan α]*d≥D1/2,

and this can be transformed into

d≥(D1*f1)/(D1+2*f1*tan α).

It is advantageous here to select a numerical value for d at least closeto the equality. The greater the remaining difference in the inequality,the more the second lens 52 comes into direct proximity with the imageplane and can there hardly still develop a useful effect.

The two lenses 50, 52 can be plano-convex, convex-plano, biconvex, andpossibly also convex-concave or concave-convex; but in the last twocases still as a converging lens. Classically refractive lenses, Fresnellenses, or diffractive optics and combinations thereof are possible. Thetwo lenses 50, 52 can differ from one another or coincide with oneanother in these general shaping properties and effective principles.The two lenses 50, 52 can have different focal lengths f1, f2, differentdiameters D1, D2, and different shapes.

In an advantageous embodiment, not only the distance between the twolenses 50, 52 is selected with reference to the above-stated inequality,but the selection f2=d′ is also made. d′ here is the distance betweenthe main planes of the lenses 50, 52 that is a little larger than thedistance d depending on the center thickness of the second lens 52.

The front focal plane of the second lens 52 is placed in the main planeof the first lens 50 at this focal length f2. This has the consequencethat the main beam runs in parallel with the optical axis in the imageplane of the objective; the objective is then therefore telecentric atthe image side. Inter alia on the use as a transmission optics 24, thismakes it possible that the light sources 22 a may be aligned in parallelwith one another and do not have to be slanted. Advantages also alreadyresult with a non-exact matching of the focal length f2 to the distanced′, that is only f2˜d′, because the main beam angle at the image side isthen already considerably reduced in size, albeit not down to 0°.

The diameter D2 of the second lens 52 is furthermore preferably onlyselected to be as large as the light beams 54, 56 that pass throughrequire. The two-lens objective is then completely determined only bythree parameters: The diameter D1 and the focal length f1 of the firstlens 50 can be freely selected. The distance d of the second lens 52results from the above-explained inequality. The focal length f2 isfinally placed at the distance d′.

A total focal length f of the objective can also be calculated fromthese now known parameters using formulas of geometrical (paraxial)optics that are known per se. Conversely, the two-lens objective canonly be fixed by its basic paraxial values: The focal length f of theobjective, the aperture D=D1 of the objective, and the field angle α ofthe circular image field.

In a further preferred embodiment with a very large, but stillachievable f-number k1:=f1/D1=1 of the first lens, the relationships aresimplified in a very graphic manner:

d=f1/(1+2 tan α), for instance with α=30°:d≈0.5*f1,

f2=d′≈d≈f1/(1+2 tan α).

All the focal lengths f1, f2 and distances d or d′ for the design of thetwo-lens objective are herewith given for this preferred embodiment forevery desired field angle α and for every desired aperture D=D1. Allthese values can again where required also be directly obtained from thedesired values f and D of the objective using the formulas known per sefor the calculation of the total focal length of two combined lenses.

Finally, a numerical example will be shown:

-   Objective focal length f=19 mm-   Aperture D=20 mm (diameter of lens 1)→k=D/f=1.05)-   First lens F2 glass: f1=29.8 mm, center thickness 4 mm, aspherically    convex-plano-   Second lens F2 glass: f2 21.6 mm, center thickness 5 mm, spherically    convex-plano-   Lens distance d=14.8 mm; distance of second lens from the image    plane: 4.2 mm-   Image field angle α=±15.4°-   Spot diameter 20 μm (=approx. 1 mrad)

1. An optoelectronic sensor for detecting objects in a monitored zone,the optoelectronic sensor comprising: at least one light transmitter fortransmitting a plurality of mutually separated light beams starting froma respective one transmission point; a transmission optics for thetransmitted light beams; at least one light receiver for generating arespective received signal from the remitted light beams reflected fromthe objects and incident at a respective reception point; a receptionoptics for the remitted light beams; and an evaluation unit foracquiring information on the objects from the received signals, whereinat least one of the reception optics and the transmission optics is atwo-lens objective for an annular image field with an image field angleα that has a first lens and a second lens, with the first lens beingconfigured such that light beams of every single transmission pointand/or reception point with an image field angle α only impinge on halfof the second lens.
 2. The optoelectronic sensor in accordance withclaim 1, wherein the inequality d≥(D1*f1)/(D1+2*f1*tan α) is satisfiedfor the two-lens objective, with a focal length f1 and a diameter D1 ofthe first lens and a distance d between the first lens and the secondlens.
 3. The optoelectronic sensor in accordance with claim 2, whereind=(D1*f1)/(D1+2*f1*tan α) applies at least approximately.
 4. Theoptoelectronic sensor in accordance with claim 1, wherein the focallength f2 of the second lens corresponds to the distance between thefirst lens and the second lens.
 5. The optoelectronic sensor inaccordance with claim 1, wherein the first lens has a small f-number k1.6. The optoelectronic sensor in accordance with claim 5, wherein thefirst lens has an f-number k1=1.
 7. The optoelectronic sensor inaccordance with claim 1, wherein the transmission points are arranged ona first circular line.
 8. The optoelectronic sensor in accordance withclaim 1, wherein the reception points are arranged on a second circularline.
 9. The optoelectronic sensor in accordance with claim 1, that hasa plurality of light transmitters.
 10. The optoelectronic sensor inaccordance with claim 9, wherein the optoelectronic sensor has one lighttransmitter per transmission point.
 11. The optoelectronic sensor inaccordance with claim 1, that has a plurality of light receivers. 12.The optoelectronic sensor in accordance with claim 11, wherein theoptoelectronic sensor has one light receiver per reception point. 13.The optoelectronic sensor in accordance with claim 1, wherein the lighttransmitter and the light receiver form a coaxial arrangement and thetransmission optics and the reception optics are combined in a commonoptics.
 14. The optoelectronic sensor in accordance with claim 1, thatis configured as a laser scanner and has a movable deflection unit withwhose aid the transmitted light beams are periodically conducted throughthe monitored zone.
 15. The optoelectronic sensor in accordance withclaim 14, wherein the deflection unit is configured in the form of arotatable scanning unit in which at least one of the light transmitterand the light receiver is accommodated.
 16. The optoelectronic sensor inaccordance with claim 1, wherein the evaluation unit is configured todetermine a distance of the objects from a time of flight between thetransmission of the light beams and the reception of the remitted lightbeams.
 17. A method of detecting objects in a monitored zone in which aplurality of mutually separated light beams are transmitted from atransmission optics starting from a respective transmission point; inwhich a respective received signal is generated from the remitted lightbeams reflected from the objects and incident at a respective receptionpoint after passing through a reception optics; and in which thereceived signals are evaluated to acquire information on the objects,wherein at least one of the reception optics and the transmission opticsis a two-lens objective for an annular image field with an image fieldangle α that has a first lens and a second lens; and wherein due to thedesign of the first lens, bundles of rays of every single transmissionpoint and/or reception point with an image field angle α only impinge onhalf of the second lens.